Various microelectronic devices require humidity levels in a range of about 2500 to below 5000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved by encapsulating the device or by sealing the device and a desiccant within a cover. Desiccants such as, for example, molecular sieve materials, silica gel materials, and materials commonly referred to as Drierite materials are used to maintain the humidity level within the above range.
Particular microelectronic devices, for example, organic light-emitting devices (OLED) or panels, polymer light-emitting devices, charge-coupled device (CCD) sensors, and micro-electro-mechanical sensors (MEMS) require humidity control to levels below about 1000 ppm and some require humidity control below even 100 ppm. Such low levels are not achievable with desiccants of silica gel materials and of Drierite materials. Molecular sieve materials can achieve humidity levels below 1000 ppm within an enclosure if dried at a relatively high temperature. However, molecular sieve materials have a relatively low moisture capacity at humidity levels at or below 1000 ppm, and the minimum achievable humidity level of molecular sieve materials is a function of temperature within an enclosure: moisture absorbed, for example, at room temperature, can be released into the enclosure or package during temperature cycling to higher temperature, such, as, for example, to a temperature of 100.degree. C. Desiccants used within such packaged devices include metal oxides, alkaline earth metal oxides, sulfates, metal halides, or perchlorates, i.e. materials having desirably relatively low values of equilibrium minimum humidity and high moisture capacity. However, such materials often chemically absorb moisture relatively slowly compared to the above-mentioned molecular sieve, silica gel, or Drierite materials. Such relatively slow reaction with water vapor leads to a measurable degree of device degradation of performance following the sealing of the desiccant inside a device cover due to, for example, moisture absorbed on the inside of a device, moisture vapor present within the sealed device, and moisture permeating through the seal between the device and the cover from the outside ambient.
Some desiccants, particularly molecular sieve materials which entrain moisture by physical absorption within microscopic pores, require a dehydrating step at substantially elevated temperature prior to use within a device enclosure, thus increasing the number of process steps and calling for additional apparatus, such as, for example, a controllable furnace to achieve substantial dehydration.
Selection of a desiccant and the method of applying a selected desiccant to an inner portion of a device enclosure prior to sealing the device within or by the enclosure is governed by the type of device to be protected from moisture. For example, highly moisture-sensitive organic light-emitting devices or polymer light-emitting devices require the selection of particular desiccants and methods of application, since organic materials or organic layers are integral constituents of such devices. The presence of organic materials or layers may, for example, preclude the use of certain solvents or fluids in the application of a desiccant dispersed in a fluid to organic-based devices. Furthermore, a thermal treatment, if required, of a desiccant contained within a sealed device enclosure, needs to be tailored to the constraints imposed by thermal properties of the organic constituents or layers of the device. At any rate, release of solvent vapors during a thermal treatment of a desiccant disposed within a sealed device enclosure must be avoided or minimized if solvent vapors can adversely affect organic constituents of organic-based electronic devices. The aforementioned considerations pertaining to organic-based electronic devices may not be as important if the electronic device to be desiccated is strictly an inorganic or metallic device such as, for example, a MEMS device or a CCD sensor without an organic color filter overlay.
Numerous publications describe methods and/or materials for controlling humidity levels within enclosed or encapsulated electronic devices. For example, Kawami et al., European Patent Application EP 0 776 147 A1 disclose an organic EL element enclosed in an airtight container which contains a drying substance comprised of a solid compound for chemically absorbing moisture. The drying substance is spaced from the organic EL element, and the drying substance is consolidated in a predetermined shape by vacuum vapor deposition, sputtering, or spinner-coating.
Shores, U.S. Pat. No. 5,304,419 discloses a moisture and particle getter for enclosures which enclose an electronic device. A portion of an inner surface of the enclosure is coated with a pressure sensitive adhesive containing a solid desiccant.
Shores, U.S. Pat. No. 5,401,536 describes a method of providing a moisture-free enclosure for an electronic device, the enclosure containing a coating or adhesive with desiccant properties. The coating or adhesive comprises a protonated alumina silicate powder dispersed in a polymer.
Shores, U.S. Pat. No. 5,591,379 discloses a moisture gettering composition for hermetic electronic devices. The composition is applied as a coating or adhesive to the interior surface of a device packaging, and the composition comprises a water vapor permeable binder which has dispersed therein a desiccant which is preferably a molecular sieve material.
Many of the desiccants disclosed by Shores will not function effectively with highly moisture-sensitive devices at a humidity level lower than 1000 ppm.
Similarly, binders, such as polyethylene disclosed by Shores, that have low moisture absorption rates compared to the absorption rate of the pure selected desiccants would not function effectively to achieve and to maintain a humidity level below 1000 ppm during a projected operational lifetime of a highly moisture-sensitive device.
Deffeyes, U.S. Pat. No. 4,036,360 describes a desiccating material that is useful as a package insert or on the interior walls of packaging boxes for applications requiring only moderate moisture protection, such as film or cameras. The material comprises a desiccant and a resin having a high moisture vapor transmission rate.
The desiccants disclosed by Deffeyes are alumina, bauxite, calcium sulfate, clay, silica gel, and zeolite. None of these desiccants will function effectively with highly moisture-sensitive devices at a humidity level lower than 1000 ppm. In addition the moisture vapor transmission rate requirement for the resin is not adequately defined since there is no reference to the thickness of the measured resins. A material that transmits 40 grams per 24 hrs per 100 in.sup.2 at a thickness of 1 mil would be very different than one that transmits 40 grams per 24 hrs per 100 in.sup.2 at a thickness of 100 mils. It is therefore not possible to determine if the moisture vapor transmission rates disclosed by Deffeyes are sufficient for highly moisture-sensitive devices.
Taylor, U.S. Pat. No. 4,013,566 describes solid desiccant bodies that are useful as drier materials in refrigerant fluid systems. The solid desiccant body comprises finely divided particles of desiccant material bound in a moisture transmissive aliphatic epoxy polymer matrix.
The desiccants disclosed by Taylor are molecular sieves, activated alumina, and silica gel. None of these desiccants will function effectively with highly moisture-sensitive devices at a humidity level lower than 1000 ppm. In addition the moisture vapor transmission rate requirement for the resin is not adequately defined; stating only that the solid desiccant bodies have rates of adsorption of absorption comparable to the desiccant materials alone. It is therefore not possible to determine if the resins disclosed by Taylor are sufficient for highly moisture-sensitive devices.
Booe, U.S. Pat. No. 4,081,397 describes a composition used for stabilizing the electrical and electronic properties of electrical and electronic devices. The composition comprises alkaline earth oxides in an elastomeric matrix.
The desiccants disclosed by Booe are barium oxide, strontium oxide, and calcium oxide. These desiccants will function effectively with highly moisture-sensitive devices at humidity levels lower than 1000 ppm; however, Booe claims the elastomeric matrix has the property of retarding the rate of fluid absorption of the alkaline earth particles. In the examples the water absorption rate of the compositions are 5 to 10 times slower than the alkaline earth particles alone. This decrease in absorption rate is disclosed as a desirable feature that improves the handling of the highly reactive alkaline earth oxides. In highly moisture-sensitive devices, however, any decrease in the absorption rate of moisture will increase the likelihood of device degradation, and identification of resins that will increase the absorption rate of moisture would be highly desirable. For highly moisture-sensitive devices, therefore, it is important to determine the minimum allowable water vapor transmission rate of the binders used in combination with effective desiccant materials.